Gas discharge tube having a paschen value less than the paschen value of gas and air



July 25, 1967 H. MUNGARD 3,333,102

GAS DISCHARGE TUBE HAVING A PASCHEN VALUE LESS THAN THE PASCHEN VALUE OFGAS AND AIR Filed Aug. 31, 1964 U MT POWER SOURCE FIG. 2.

United States Patent 3,333,102 GAS DISCHARGE TUBE HAVING A PASCHEN VALUELESS THAN THE PASCHEN VALUE OF GAS AND AIR Hans Mungard, Langeso,Denmark, assignor to Dantoss A/S, Nordborg, Denmark, a company ofDenmark Filed Aug. 31, 1964, Ser. No. 393,163 5 Claims. (Cl. 250-83.6)

This invention relates generally to radiation detectors and moreparticularly to gas-filled detector tubes which operate according toGeiger-Muller principles.

Radiation detectors, in the form of gas-filled detector tubes, in whichan ionizable gas is contained in a radiation pervious chamber containingthe gas and in which electrodes are disposed and to which is applied aspecific or selected working voltage are well known. In these devicesthe radiation ionizes the gas in the tube such that electron flowresults and the tube is rendered conductive or fires. Such tubes areused for detecting electromagnetic radiation having wave lengths in theregion of the electromagnetic spectrum comprising ultraviolet radiation,for example, in automatic controls for sensing a flame in heaters and infire alarms.

The known devices or tubes, however, do not always operate accuratelysince at times they are rendered conductive and indicate the presence ofradiation when none exists. This may result after a period of time whenthe tube has been installed or when the normal applied voltage isapplied thereto even in the absence of radiation. This kind of improperoperation may result, for example, in the actuation of a fire alarm whenthere is actually no fire or in heaters, having such devices, when theflame is out and the device can continue the operation of an oil pumpand the delivery of oil to a burner even when the flame is out.

A principal object of the present invention is to provide a gas-filledradiation detector tube, for detecting electromagnetic radiation in theregion of the spectrum comprising ultraviolet radiation, which issubstantially, absolutely reliable in its operation.

A feature of the detector tubes according to the invention is a tubeconstruction in which the minimum value of a Paschen curve of the gas islower than the minimum value of either a Paschen curve of air or of agas-air mixture. It is known that miscroscopically fine leaks, exist intubes sometimes allowing the entry of air thereby resulting in a Paschencurve change which can result in misoperation of the known tubes. Ifinitially the detector tubes are operated on an ignition or firing pointon the descending or negative branch of the Paschen curve, then theignition or firing voltage level is lowered by the entry of air. Thus,self-ignition can occur with normal operating voltage applied to thetube.

According to the invention, the selection of the relative minimum valuesof the Paschen curves of the gas and the air of gas-air mixture is suchthat the gas Paschen curve minimum value is below the minimum value ofthe other curves and thereby ensures safe operation. Thus by selectingthe working voltage, the type of gas and the product of the gas pressureand the distance between the electrodes resulting in such a Paschencurve the device according to the invention will detect ultravioletradiation as desired and will not fire in the absence of the presence ofradiation.

Another feature according to the invention is a gasfilled detector tubeconstruction in which the original datum or point of ignition is chosento lie on the rising or positive branch of the Paschen curve of the gasand the gas-air mixture curve.

Other features and advantages of the radiation detection tube inaccordance with the present invention will be "ice better understood asdescribed in the following specification and appended claims, inconjunction with the following drawing in which:

FIG. 1 is a diagram of Paschen curves for illustrating the principles ofthe invention; and 7 FIG. 2 is a diagrammatic illustration of a detectortube according to the invention.

Those skilled in the art know that Paschens Law states that the sparkingpotential between two parallel-plate electrodes in a gas is proportionalto the product of gas pressure and electrode spacing. According to thediagram or plot in FIG. 1, the ordinate U (v) designates the appliedvoltage values and the abscissa in the diagram is representative of theproduct, p d, of the air pressure p in the interior of an ultravioletdetector tube and the distance d between the electrodes. In the curvesillustrated, the curve G is a Paschen curve of a substantially pure gasand the curve L is the Paschen curve for air. These Paschen curves areknown for certain gases and may be found in standard treatises.

From the plot or diagram in FIG. 1 it can be seen that the self-ignitionvoltage of the gas can change very rapidly if it lies on the negativebranch of the gas curve in view of the rapidly descending curve of theproduct p xel increases and therefore it can easily fall below anormally applied voltage if the gas pressure in the interior of the tubeis increased by the addition of air, for example, thereby resulting inan increase in the product pxd and firing of the tube in the absence ofionizing radiation. If gas-filled tubes worked properly and wereabsolutely airtight, a detector tube might be constructed with anignition point chosen far up on the descending or negative branch of thePaschen curve of the gas and the working or applied voltage could be ata level between this point and the minimum value of the curve. However,as indicated heretofore, the entrance of air does result and the tubesconstructed in this manner will misoperate.

The diagram on FIG. 1 illustrates two examples of constructions whichare used according to the invention for constructing detector tubes.According to the invention the gas Paschen curve 6 has a minimum valuebelow the value of the air Paschen curve L and self-ignition voltage Pis selected relative to an applied voltage U and the product of the gaspressure and the distance between the electrodes. In the event that insuch a construction air enters the tube the Paschen curve, the curve Gof the gas, changes gradually into the Paschen curve values of air sothat the resulting mixture curve is illustrated as a curve U which isrepresentative of the transition between a relatively pure gas to agas-air mixture. The working voltage or applied voltage U is below theminimum value of the curve G so that an intersection of the Paschencurve with the working voltage cannot occur and the tube will not fireunless radiation is sensed and the gas ionized. As may be seen from thegraph or diagram, the applied voltage U may even be raised a little to ahigher value since the curve U is not a descending curve but a risingcurve which, at first, is substantially a flat horizontal curve and thenrises gradually. Thus by constructing a tube in which the gas has aPaschen curve minimum value below the minimum value of the Paschen curveof air, as a function of the product of the gas pressure and thedistance between the electrodes selected for such a tube and resultingin such a curve if a working voltage is applied as above the result is atube that will detect the presence of ultraviolet radiation and stillwill not fire or be rendered conductive in the absence thereof.

According to the invention, another construction is possible in whichthe firing or ignition voltage point P is chosen to lie on the rising orpositive branch of the Paschen curve of the gas. In the event that thetube has a minor leak, the entering air will cause the gas Paschenlected and applied at a value far above the minimum value of the Paschencurve G. The rapidly rising curve U will preclude self-ignition in theabsence of ionization resulting from detection or sensing of ionizingradiation.

A detector tube constructed according to the invention is illustrated inFIG. 2, in which the tube comprises means which may be, for example, anenvelope 1 which is pervious to ultraviolet radiation or at least has aportion thereof which is a window and pervious to this radiationdefining a gas chamber 3 in which is contained an ionizable gas. Thetube comprises two parallel spaced electrodes 4 and 5 having pins 6 and7 connected thereto respectively for connecting the electrodes to apower voltage source as illustrated by which the selected operatingvoltage is applied. Upon the incidence of ultraviolet radiation, on thegas chamber 3 the gas is ionized and the tube will ignite when theignition point is reached as above described.

While preferred embodiments of the invention have been shown anddescribed, it will be understood that many modifications and changes canbe made within the true spirit and scope of the invention.

What I claim and desire to secure by Letters Patent is:

1. A gas-filled detector tube for detecting electromagnetic radiationhaving wave lengths in the region of the electromagnetic spectrumcomprising ultraviolet radiation, said tube comprising, means defining agas chamber having at least a portion pervious to electromagneticradiation having wave lengths comprising ultraviolet radiation, anionizable gas in said chamber ionizable by said radiation, two spaced,parallel electrodes internally of said chamber, means connected to saidelectrodes for receiving a selected voltage for application to saidelectrodes in operation for firing said tube when said radiation ionizessaid gas, the product of the pressure of said gas and the distancebetween electrodes of said tube being selected and represented by aPaschen curve having a minimum value less than the minimum valuerepresented by a Paschen curve of a mixture of said gas and air, andsaid voltage having a selected value relative to level of the minimumvalue of said gas Paschen curve.

2. A gas-filled detector tube according to claim 1, in which saidvoltage has a value below the minimum value of the Paschen curve of saidgas.

3. A gas-filled detector tube according to claim'l, in which saidvoltage has a value above the minimum value of said Paschen curve, andin which the point of ignition is on a rising portion of said gasPaschen curve.

4. A gas-filled detector tube for detecting electromagnetic radiationhaving wave lengths in the region of the electromagnetic spectrumcomprising ultraviolet radiation, said tube comprising, means defining agas chamber having at least a portion pervious to electromagneticradiation having wave lengths comprising ultraviolet radiation, anionizable gas in said chamber ionizable by said radiation, two spaced,parallel electrodes internally of said chamber, means connected to saidelectrodes for receiving a selected voltage for application to saidelectrodes in operation for firing said tube only when said radiationionizes said gas, the product of the pressure of said gas and thedistance between electrodes of said tube being selected and representedby a Paschen curve of a mixture of air and said gas having a minimumvalue above the level of the selected voltage.

5. A gas-filled detector tube for detecting electromagnetic radiationhaving wave lengths in the region of the electromagnetic spectrumcomprising ultraviolet -radia-' tion, said tube comprising, meansdefining a gas chamber having at least a portion pervious toelectromagnetic radiation having wave lengths comprising ultravioletradiation, an ionizable gas in said chamber ionizable by said radiation,two spaced, parallel electrodes internally of said chamber, meansconnected to said electrodes for. applying a selected voltage to saidelectrodes in operation for firing said tube when said radiation ionizessaid gas to a given level of ionization, the product of the pressure ofsaid gas and the distance between electrodes of said tube being selectedand represented by a Paschen curve having a minimum value less than theminimum value represented by a Paschen curve of a mixture of said gasand air, and said voltage having a selected value relative to theminimum value of said gas Paschen curve.

References Cited UNITED STATES PATENTS 1,481,422 1/1924 Holst et al.3l3-226 1,656,957 1/1928 Schroter 313-226 X 3,213,312 10/1965 Crowe etal 25083.3 X 3,237,041 2/1966 Barnes 313-226 X OTHER REFERENCES Millman,1.: Vacuum-tube and Semiconductor Electronics, 1958, pp. 29930l reliedon.

RALPH G. NILSON, Primary Examiner. S. ELBAUM, Assistant Examiner.

1. A GAS-FILLED DETECTOR TUBE FOR DETECTING ELECTROMAGNETIC RADIATIONHAVING WAVE LENGTHS IN THE REGION OF THE ELECTROMAGNETIC SPECTRUMCOMPRISING ULTRAVIOLET RADIATION, SAID TUBE COMPRISING, MEANS DEFINING AGAS CHAMBER HAVING AT LEAST PORTION PERVIOUS TO ELECTROMAGNETICRADIATION HAVING WAVE LENGTHS COMPRISING ULTRAVIOLET RADIATION, ANIONIZABLE GAS IN SAID CHAMBER IONIZABLE BY SAID RADIATION, TWO SPACED,PARALLEL ELECTRODES INTERNALLY OF SAID CHAMBER, MEANS CONNECTED TO SAIDELECTRODES FOR RECEIVING A SELECTED VOLTAGE FOR APPLICATION TO SAIDELECTRODES IN OPERATION FOR FIRING SAID TUBE WHEN SAID RADIATION IONIZESSAID GAS, THE PRODUCT OF THE PRESSURE OF SAID GAS AND THE DISTANCEBETWEEN ELECTRODES OF SAID TUBE BEING SELECTED AND REPRESENTED BY APASCHEN CURVE HAVING A MINIMUM VALUE LESS THAN THE MINIMUM VALUEREPRESSENTED BY A PASCHEN CURVE OF A MIXTURE OF SAID GAS AND AIR, ANDSAID VOLTAGE HAVING A SELECTED VALUE RELATIVE TO LEVEL OF THE MINIMUMVALUE OF SAID GAS PASCHEN CURVE.